MRI systems acquire diagnostic images without relying on ionizing radiation. Instead, MRI employs strong, static magnetic fields, radio-frequency (RF) pulses of energy, and time varying magnetic field gradient waveforms. Unfortunately, the strong, static magnetic fields may sometimes experience temporal, spatial, field strength, and/or other variations, which may impact imaging applications that rely on proton resonant frequency shifting and/or other applications (e.g., velocity measurement) using phase shifting.
MRI is a non-invasive procedure that employs nuclear magnetization and radio waves to produce internal pictures of a subject. Two or three-dimensional diagnostic image data is acquired for respective “slices” of a subject area. These data slices typically provide structural detail having, for example, a resolution of one millimeter or better. Programmed steps for collecting data, which is used to generate the slices of the diagnostic image, are known as an MR image pulse sequence. The MR image pulse sequence includes generating magnetic field gradient waveforms applied along up to three axes, and one or more RF pulses of energy. The set of gradient waveforms and RF pulses are repeated a number of times to collect sufficient data to reconstruct the image slices.
Data is acquired during successive repetitions of an MR imaging pulse sequence or excitation. Ideally, there is little or no variation in the nuclear magnetization and the spatio-temporal characteristics of the background magnetic field during the respective excitations. However, variations can occur. When variations occur, data used to create an image between respective excitations may have peak signal locations that become misaligned. Thus, the nuclear magnetization variations may degrade the quality of the MR data used to produce the images, particularly in PRF shift applications.
Sources of background phase variation can dominate the features of phase images used to generate temperature difference maps in PRF MR thermometry. This is particularly problematic at low magnetic field strengths (e.g., 0.2T resistive magnets). These errors exist, albeit to a lesser extent, when performed on higher field and/or superconducting systems.